A resin component for a masterbatch of foaming agent must have a good compatibility with a matrix resin used and be kneaded uniformly in the state of not decomposing a foaming agent. It is therefore preferred that the crystalline resin component has a low melting point, the amorphous resin component has a low glass transition point and also both the crystalline and amorphous resin components have a low melt viscosity.
For example, in the case of using a polyolefin as the matrix resin, an ethylene-based resin is used as the resin component for masterbatch of foaming agent and the representative examples thereof are a high density polyethylene, a low density polyethylene, a linear low density polyethylene, an ethylene-vinyl acetate copolymer, etc.
In the case of obtaining a foamed product of styrene-based resin, a syrenic resin such as a polystyrene or an acrylonitrile-butadiene-styrene copolymer is used as the resin component for the masterbatch of foaming agent.
In addition, in the case of obtaining a foamed product of a polycarbonate, a modified polyphenylene oxide, 6-nylon, a polybutyrene terephthalate, a polyethylene terephthalate or the like, a polystyrene or an acrylonitrile-butadiene-styrene copolymer is generally used as the resin component for the masterbatch of foaming agent.
However, in particular, the masterbatch of foaming agent for a foamed product of the styrenic resin, polycarbonate, modified polyphenylene oxide, 6-nylon, polybutylene terephthalate and polyethylene terephthalate uses a styrenic resin as a base polymer thereof and, therefore, there are the following problems.
Since the melt viscosity of a polymer is high, the kneadability of a polymer is extremely poor (the melt flow index according to JIS K6870 is 30 g/10 min in the polystyrene and 33 g/10 min in the acrylonitrile-butadiene-styrene copolymer). For instance, when a polymer and a foaming agent are kneaded with an extruder to prepare a masterbatch of foaming agent, even if the kneading is conducted at a temperature sufficiently lower than the thermal decomposition temperature of the foaming agent, a heat generation by shearing occurs due to the high melt viscosity of a resin and as a result, the foaming agent is decomposed. In order to prevent this, if the kneading is conducted at a lower temperature, the screw torque remarkably increases and the rotation of screw becomes impossible. Further, even when roll or Bambury mixer is used for kneading, the heat generation by shearing becomes large and when the concentration of the foaming agent added is high, it is difficult to control decomposition of the foaming agent.